1. Field of the Invention
The present invention relates to a process for the production of dialkyl carbonates, particularly dimethyl carbonate wherein the reaction occurs simultaneously with separation of the reactants and the carbonate products. More particularly the invention relates to a process wherein methanol is reacted with urea and/or alkyl carbamate in the presence of a complex compound catalyst comprising an organic tin compound and an electron donor oxygen atom containing compound.
2. Related Art
Dialkyl carbonates are important commercial compounds, the most important of which is dimethyl carbonate (DMC). Dimethyl carbonate is used as a methylating and carbonylating agent. It can also be used as a solvent to replace halogenated solvents such as chlorobenzene. Although the current price of dimethyl carbonate is prohibitively expensive to use as fuel additive, it could be used as an oxygenate in reformulated gasoline and an octane component. Dimethyl carbonate has a much higher oxygen content (53%) than MTBE (methyl tertiary butyl ether) or TAME (tertiary amyl methyl ether), and hence not nearly as much is needed to have the same effect. It has a RON of 130 and is less volatile than either MTBE or TAME. It has a pleasant odor and, unlike ethers, is more readily biodegradable.
In older commercial processes dimethyl carbonate was produced from methanol and phosgene. Because of the extreme toxicity and cost of phosgene, there have been efforts to develop better, non-phosgene based processes.
In one new commercial process, dimethyl carbonate is produced from methanol, carbon monoxide, molecular oxygen and cuprous chloride via oxidative carbonylation in a two step slurry process. Such a process is disclosed in EP 0 460 735 A2. The major shortcomings of the process are the low production rate, high cost for the separation of products and reactants, formation of by-products, high recycle requirements and the need for corrosion resistant reactors and process lines.
Another new process is disclosed in EP 0 742 198 A2 and EP 0 505 374 B1 wherein dimethyl carbonate is produced through formation of methyl nitrite instead of the cupric methoxychloride noted above. The by-products are nitrogen oxides, carbon dioxide, methylformate, etc. Dimethyl carbonate in the product stream from the reactor is separated by solvent extractive distillation using dimethyl oxalate as the solvent to break the azeotropic mixture. Although the chemistry looks simple and the production rate is improved, the process is actually very complicated because of the separation of a number of the materials, balancing materials in various flow sections of the process, complicated process control and dealing with methyl nitrite, a hazardous chemical.
In another commercial process dimethyl carbonate is produced from methanol and carbon dioxide in a two step process. In the fist step cyclic carbonates are produced by reacting epoxides with carbon dioxide as disclosed in U.S. Pat. Nos. 4,786,741; 4,851,555 and 4,400,559. In the second step dimethyl carbonate is produced along with glycol by exchange reaction of cyclic carbonates with methanol. See for example Y. Okada, et al xe2x80x9cDimethyl Carbonate Production for Fuel Additivesxe2x80x9d, ACS, Div. Fuel Chem., Preprint, 41(3), 868, 1996, and John F. Knifton, et al, xe2x80x9cEthylene Glycol-Dimethyl Carbonate Cogenerationxe2x80x9d, Journal of Molecular Chemistry, vol. 67, pp 389-399, 1991. While the process has its advantages, the reaction rate of epoxides with carbon dioxide is slow and requires high pressure. In addition the exchange reaction of the cyclic carbonate with methanol is limited by equilibrium and methanol and dimethyl carbonate form an azeotrope making separation difficult.
It has been known that dialkyl carbonates can be prepared by reacting primary aliphatic alcohols such as methanol with urea in the presence of various heterogeneous and homogeneous catalysts such as dibutyltin dimethoxide, tetraphenyltin, etc. See for example P. Ball et al, xe2x80x9cSynthesis of Carbonates and Polycarbonates by Reaction of Urea with Hydroxy Compoundsxe2x80x9d,C1 Mol. Chem. vol. 1, pp 95-108, 1984. Ammonia is a coproduct and it may be recycled to urea as in the following reaction sequence. 
Carbamates are produced at a lower temperature followed by production of dialkyl carbonates at higher temperature with ammonia being produced in both steps. 
As noted the above two reactions are reversible under reaction conditions. The order of catalytic activity of organotin compounds is R4Sn less than R3SnX less than  less than R2SnX2, wherein X=Cl, RO, RCOO, RCOS. A maximum reaction rate and minimum formation of by-products are reported for dialkyl tin (IV) compounds. For most catalysts (Lewis acids), higher catalyst activity is claimed if the reaction is carried out in the presence of an appropriate cocatalyst (Lewis base). For example, the preferred cocatalyst for organic tin (IV) catalysts such as dibutyltin dimethoxide, dibutyltin oxide, etc. are triphenylphosphine and 4-dimethylaminopyridine. However, the thermal decomposition of intermediate carbamates to isocyanic acid (HNCO) or isocyanuric acid ((HNCO)3) and alcohols is also facilitated by the organotin compounds such as dibutyltin dimethoxide or dibutyltin oxide employed in the synthesis of aliphatic carbamates. WO 95/17369 discloses a process for producing dialkyl carbonate such as dimethyl carbonate in two steps from alcohols and urea, utilizing the chemistry and catalysts published by P. Ball et al. In the first step, alcohol is reacted with urea to produce an alkyl carbamate. In the second step, dialkyl carbonate is produced by reacting further the alkyl carbamate with alcohol at temperatures higher than the first step. The reactions are carried out by employing an autoclave reactor. However, when methanol is reacted with methyl carbamate or urea, N-alkyl by-products such as N-methyl methyl carbamate (N-MMC) and N-alkyl urea are also produced. The dialkyl carbonate is present in the reactor in an amount between 1 and 3 weight % based on total carbamate and alcohol content of the reactor solution to minimize the formation of the by-products.
In U.S. Pat. No. 5,902,894, dimethyl carbonate (DMC) is synthesized from urea and methanol in high yield in a single step in the presence of high boiling ethers and a novel homogeneous tin complex catalyst.
(NH2)2CO+2CH3OHxe2x86x92(CH3O)2CO+2NH3↑
The ether solvent also serves as complexing agent to form the homogenous complex catalyst from dibutyltin dimethoxide or oxide in situ.
The separation of materials involved in the DMC processes is very important for the commercial production of DMC for economic reasons. EP 0 742 198 A1 and U.S. Pat. No. 5,214,185 disclose the separation of DMC from a vapor mixture of methanol and DMC by using dimethyl oxalate(DMOX) as extraction solvent. Because of the high melting point of DMOX (54xc2x0 C.), using DMOX is inconvenient and adds an extra cost to the separation. In the present invention a new material has been found to provide for a superior separation. Further, a novel method is disclosed for conducting the reaction continuously and with the simultaneous separation of the DMC product from the reaction mixture.
Dialkyl carbonates are prepared by reacting alcohols with urea or alkyl carbamate or both in the presence of a dibutyltin dimethoxide complex compound and high boiling oxygen containing organic solvent wherein the reaction is preferably carried out in the reboiler of a distillation still with concurrent distillation of the dialkyl carbonate. The complexing agent and the solvent are preferably the same compound. But they can be different oxygen containing organic compounds, if chosen to obtain the best performing environment to produce dialkyl carbonates such as dimethyl carbonate or diethyl carbonate. The concentration of homogeneous catalyst in the reaction zone is from 2 to 50%, preferably 3 to 40%. The concentration of electron donor oxygen containing solvent in the reaction zone is from 0.01% to 80%, preferably 3 to 65%.
In a preferred embodiment the complexing agents have one, two, three, four or more oxygen atoms per molecule, preferably two or more oxygen atoms per molecule preferably polyglycol ethers such as triglyme (triethylene glycol dimethyl ether), whose boiling point is higher than either methanol or dimethyl carbonate and which serve as both cocatalysts and solvent. In another preferred embodiment reaction is conducted continuously in a multistage distillation column where urea dissolved in methanol is introduced some number of stages above the reboiler, methanol vapor is introduced into the reboiler, and complex compound catalyst solution collected in the reboiler is circulated from the reboiler to some stage below from the methanol-urea feed stage via a pump. The downflowing reaction mixture is thus counter currently contacted with upflowing methanol vapor which strips the product DMC and ammonia from the reaction mixture. The column is configured with a reflux condenser and a number of stages above the methanol-urea feed stage. The column is operated with sufficient reflux vapors back down the column thus maintaining the catalyst solution inventory in the column.
Diethyl oxalate (DEOX) is preferably used as an extractive distillation solvent to separate DMC from the low-pressure azeotrope of DMC and methanol. DEOX has low melting point (xe2x88x9240xc2x0 C.). Also it has higher boiling point (185.7xc2x0 C.) than DMOX (164.5xc2x0 C). In this utilization DEOX has more desirable properties than DMOX.
Also in the present process a small amount of amine, e.g., methylamine (MA) (b.p. 48xc2x0 C.) may be produced. The MA and ammonia in the overhead stream from the catalytic distillation column is separated from the rest of the components in the stream by using a separator as overhead stream. Ammonia in the overhead stream from the separator is recovered as overhead product from the ammonia recovery column. MA is removed from the ammonia recovery column as bottom stream (organic waste). The bottom stream from the separator is composed of methanol and DMC, from which product DMC is separated as final pure product as shown later in FIG. 7.
Thus, the present invention provides an improved process by concurrently distilling dialkyl carbonate away from the reaction concurrently with the reaction and by preferably using specific complexing agents for the organotin complex catalyst. Since water is not coproduced, the reaction mixture (the overhead product) does not form a ternary azeotrope and, hence the separation of the product dimethyl carbonate from the overhead mixture is easier than the current commercial processes which have to deal with such a ternary azeotrope or prevent forming a ternary azeotrope.